Any idea what Mac this is?

According to your attached picture it's old desktop monitor. I can clearly remember when I learnt computer first I used Samsung. But not clearly say what brand is this one.

 

That is one of the last PowerMac G4s that were made. Known as the Mirror Drive Door or MDD. There were 3 variations released in 2002-3. The monitor is one of the early Cinema displays. Probably from the same time as the MDD.

 

The newest version of OS X it can run is 10.5.8.

 

That is Leopard:https://en.wikipedia.org/wiki/Mac_OS_X_Leopard

 

Mac OS X 10.5.8 = Leopard. From that image, it is not possible to tell if that iMac is a 2005 or a 2006 iMac.

 

When I bought my current Mac it came with snow leopard in late 2012. I also bought the latest (then) OS X - Mountain Lion. Never regretted it.

 

@Paulk A late 2012 rMBP would have come with Mountain Lion originally, not Snow Leopard. No rMBP can even boot Snow Leopard, except in a VM.

The PowerMac is stuck on 10.5.8 Leopard, it won't run anything after that because it is PowerPC-based. If the iMac is Intel-based (and not a last-gen G5) it will be possible to update further, but how far depends on if the CPU. Snow Leopard is the max for a Core Duo (although some have modified Lion to run on one).

 

Thanks for that, I'll take your word for it.